import std.stdio;
import std.string;

import chunk;
import parser;
//import value;
//import obj;
import dbg;

enum InterpretResult {
    OK,
    COMPILE_ERROR,
    RUNTIME_ERROR,
}

class VM {
    int topOffset = 0;
    ubyte ip = 0;
    Value[] stack;
    //ObjFunction func;
    Function func;

    this(Function func) {
        this.func = func;
    }
    
    Value peek(int offset) {
        if (offset >= this.topOffset) {
            writefln("offset of %d greater than stack size %d", offset, topOffset);
        }
        return this.stack[topOffset - offset - 1];
    }

    Value pop() {
        if (this.topOffset > 0) {
            this.topOffset--;
        } else {
            writeln("pop() on an empty stack!!");
        }
        return this.stack[this.topOffset];
    }

    //InterpretResult push(Value value) {
    void push(Value value) {
        if (this.stack.length > this.topOffset) {
            this.stack[topOffset] = value;
        } else {
            this.stack ~= value;
        }
        this.topOffset++;
    }

    ubyte readByte() {
        return this.func.chunk.code[this.ip++];
    }

    bool isNumber(Value value) {
        return value.type == ValueType.NUMBER;
    }

    double asNumber(Value value) {
        return value.as.number;
    }

    InterpretResult run() {
        //int ip = 0;  // TODO this is wrong
        while (true) {
            write("          ");
            //foreach (Value val; this.stack) {
            for (int i = 0; i < this.topOffset; i++) {
                writef("[ %s ]", printableValue(this.stack[i]));
            }
            writeln("\n--");
            disassemble(this.func.chunk, this.ip);
            ubyte inst;
            switch (inst = this.readByte()) {
                case OpCode.OP_CONSTANT:
                    Value constant = this.func.chunk.constants[this.readByte()];
                    this.push(constant);
                    break;
                case OpCode.OP_NEGATE:
                    if (!isNumber(peek(0))) {
                        writeln("negate operand must be a number");
                        return InterpretResult.RUNTIME_ERROR;
                    }
                    double val = asNumber(pop());
                    push(makeNumberValue(val * -1));
                    break;
                case OpCode.OP_ADD:
                    Value b = this.pop();
                    if (!isNumber(b)) {
                        writeln("b is not a number!");
                        return InterpretResult.RUNTIME_ERROR;  // TODO error
                    }
                    Value a = this.pop();
                    if (!isNumber(a)) {
                        writeln("a is not a number!");
                        return InterpretResult.RUNTIME_ERROR; // TODO error
                    }
                    double bnum = asNumber(b);
                    double anum = asNumber(a);
                    push(makeNumberValue(anum + bnum));
                    break;
                case OpCode.OP_SUBTRACT:
                    Value b = this.pop();
                    if (!isNumber(b)) {
                        writeln("b is not a number!");
                        return InterpretResult.RUNTIME_ERROR;  // TODO error
                    }
                    Value a = this.pop();
                    if (!isNumber(a)) {
                        writeln("a is not a number!");
                        return InterpretResult.RUNTIME_ERROR; // TODO error
                    }
                    double bnum = asNumber(b);
                    double anum = asNumber(a);
                    push(makeNumberValue(anum - bnum));
                    break;
                case OpCode.OP_LESS:
                    Value b = this.pop();
                    if (!isNumber(b)) {
                        writeln("b is not a number!");
                        return InterpretResult.RUNTIME_ERROR;  // TODO error
                    }
                    Value a = this.pop();
                    if (!isNumber(a)) {
                        writeln("a is not a number!");
                        return InterpretResult.RUNTIME_ERROR; // TODO error
                    }
                    double bnum = asNumber(b);
                    double anum = asNumber(a);
                    push(makeBooleanValue(anum < bnum));
                    break;
                case OpCode.OP_RETURN:
                    Value ret = this.pop();
                    writefln("returned %s", printableValue(ret));
                    return InterpretResult.OK;
                default:
                    writeln("unknown opcode to run");
                    break;
            }
        }

    }

}